Recent developments in liquid MHD systems have made them feasible for use in electric engines or areas in which there is no main power grid or the cost of delivery of electric power is high. They can also be used as mobile electric power generators. In general, a magnetohydrodynamic (MHD) system uses a liquid metal (LM) moving through a magnetic field to produce electricity.
A Hartmann duct 10, which is known in the art and is shown in FIG. 1, can be employed as a generator. The Hartmann principle is further set forth in "Steady Motion of Conducting Fluids in Pipes Under Transverse Magnetic Fields," J. A. Shercliff, Cambridge Phil. Soc., Vol.49, 136-144 (1953). A Hartmann duct 10 is a rectangular conduit with insulating plates 20 on two sides perpendicular to the direction of an applied magnetic field. Conducting plates in the form of a negative electrode 22 and a positive electrode 23 are on the two remaining sides that are parallel to the magnetic field. Liquid metal flows through the Hartmann duct 10 in one direction. A standard Hartmann duct outputs direct-current electric power with numerical voltages typically on the order of a few on-thousandths part of the numerical value of the current in amperes. This volt-amperage ratio is not practical for usage in electrical devices. The MHD device of the present invention overcomes this deficiency.
PCT application WO 88/05223 to Sainsbury et al. teaches a Reciprocating Free Liquid Metal Piston Stirling Cycle Linear Synchronous Generator. This reference dismisses the idea of using MHD devices in generators and engines because the Hartmann duct, which is a single duct device, is inefficient and results in a low voltage, high current device which is not practical.
Sainsbury further dismisses MHD devices on the basis that they are not particularly efficient due to the relatively high resistivity of liquid metal, even liquid sodium, compared with solid copper. An examination of the theory of liquid metal MHD devices shows that the higher specific resistivity of liquid metal compared to that of copper can be easily compensated for by enlarging the duct so that the cross-sectional area of the flowing electric current multiplied by the specific resistivity of the liquid metal results in a total resistivity that can be as low or lower than that of a copper wire selected for a winding in a solid metal generator with the same output requirements. The current-carrying cross-sectional area of the MHD duct can be increased by either increasing the height (magnetic gap) or the length of the duct.
In addition, Sainsbury also dismisses MHD devices because of eddy currents which occur where the liquid metal enters and leaves magnetic field. The technique of eliminating these eddy currents, also called "end-zone shorting" or "end-current loops", has been solved in the past by inserting nonconducting vanes in the end zones that do not interfere with the flow of liquid metal. This technique has been described by Sutton, Hurwitz, and Poritsky in "Electrical and Pressure Losses in a Magnetohydrodynamic Channel Due to End Current Loops", IEEE Transactions, January 1962, Vol. 58, pp 687-689.
A further difficulty with typical Hartmann duct MHD devices, which is not addressed by Sainsbury, results from the magnetic field induced by the generated electric current. Referring to FIG. 1A, the electric current vector j is directed transverse to the liquid metal velocity vector v and transverse to the steady imposed magnetic field B.sub.0, according to the vector cross product v.times.B.sub.0 of the liquid velocity vector and the imposed magnetic field vector. The induced magnetic field H encircles the electric current flow according to the right-hand rule. Thus the induced magnetic field lines are directed parallel to the fluid flow, in a longitudinal direction. When the MHD electric current is large, the induced magnetic field causes unstable lateral pressures and vortices in the fluid flow. For Hartmann duct MHD devices generating useful power, the current is large, and it has been found to be necessary to eliminate these instabilities by "backstrapping." Backstrapping is accomplished by a heavy copper electrode which carries the current from one electrode of the Hartmann duct over the top of one of the insulating plates to the opposite electrode. The induced magnetic field due to the electric current in this strap is equal to and opposite the induced magnetic field due to the electric current in the liquid metal, thus effectively canceling the longitudinal magnetic field and eliminating instabilities. The MHD device in the present invention does not require backstrapping because the double duct configuration with counterflow liquid in adjacent ducts produces induced magnetic fields that exactly cancel each other.
Free-piston linear motion two-stroke engines having solid metallic alternators to generate electricity are disclosed in U.S. Pat. No. 4,876,991 to Galitello, Jr. However, this reference suffers reduction in fuel conversion efficiency at power outputs less than the maximum and the range of power output variation will be many times less than the liquid metal engine. This reference also uses a heavy solid metal plunger to generate electricity and must be suspended by bearings or flexible suspension. In a liquid metal device the need for bearings is eliminated because the moving material is liquid.
An example of a double duct liquid metal electrical generator using an MHD device is shown in U.S. Pat. No. 5,473,205 to Haaland, incorporated by reference herein. However, double-duct engines use two separate channels of liquid metal, and therefor, require four combustion chambers, four combustion pistons, four magnetic couplings and four liquid metal pistons. This results in a need for synchronization devices between the two liquid metal channels.